1. Field of the Invention
The present invention relates to an image forming apparatus, more specifically, an image forming apparatus wherein an image is formed by electrophotographic processing.
2. Description of Related Art
In general electrophotographic processing wherein a laser beam or an LED is utilized as a print head, an image development is based on the principle of reversal development because an electrostatic latent image is formed as a negative document's image.
In such electrophotographic processing, as shown in FIG. 14a, an image is exposed to a photosensitive layer with surface potential (Vo) by said print head to form a negative electrostatic latent image, and thereafter with a developing bias of potential (Vb) impressed on a developing sleeve, the reversal development is performed by a developer composed of two elements. As a result, toner sticks to the parts shown by oblique lines in FIG. 14a, and a positive toner image can be formed. Image widths a and a' to be reproduced at that time almost correspond to the cross sectional widths of the developing bias potential (Vb).
Recently, in reproducing images, quality and accuracy are intended to be regarded more, so that an electrophotographic printer which can reproduce images with narrow and sharp line widths is required. As mentioned above, however, In the way that the developing bias of potential (Vb) almost equal to the initial surface potential (Vo) of the photosensitive layer is impressed, the image widths a and a' are large by all means, and it is difficult to sharpen line images.
As a measure to cope with the drawback, first, the diameter of the beam from the print head should be reduced, but this may result in an increase of cost and therefore this is not a desirable measure. Also, if the diameter of the beam is reduced, such side effects as a lag of mechanical system, uneven drive, etc. which may affect images will occur.
On the other hand, to attain narrow line widths by any treatment in processes after the image exposure, as shown in FIG. 14b, it will be possible to gain the difference between the developing bias potential (Vb) and the surface potential (Vo). FIG. 14b shows an example that the developing bias potential (Vb) is largely lowered. Thereby, image widths b and b' to be reproduced become narrower than the image widths a and a' shown in FIG. 14a although they are the same latent images. Taking this way when using a developer composed of carriers and toner, however, will cause a problem that the carriers charged with the polarity (positive polarity in FIG. 14b) opposite to the surface potential (Vo) stick to non-imaged portions with high potential. The deposition of the carriers on the photosensitive layer will cause such inconveniences as poor transference, the occurrence of flaws of the photosensitive layer at a cleaning section, uneven development on account of the loss of the developer in a developing device, etc. Such inconveniences are obvious when small diameter carriers are used in order to improve quality of images or when binder type carriers wherein magnetic powder is spread among resin are used.
If the initial surface potential (Vo) is reduced synchronized with the reduction of the developing bias potential (Vb), as shown in FIG. 14c, the difference between the potential (Vo) and the potential (Vb) will not become larger, and accordingly the deposition of the carriers on the non-imaged portions will not occur. However, image widths c and c' shown in FIG. 14c are larger than the image widths b and b' and almost as large as the image widths a and a' shown in FIG. 14a, the ones before taking a measure.